X-ray Spectroscopic Characterization of Co(IV) and Metal–Metal Interactions in Co₄O₄: Electronic Structure Contributions to the Formation of High-Valent States Relevant to the Oxygen Evolution Reaction

Abstract

The formation of high-valent states is a key factor in making highly active transition-metal-based catalysts of the oxygen evolution reaction (OER). These high oxidation states will be strongly influenced by the local geometric and electronic structures of the metal ion, which are difficult to study due to spectroscopically active and complex backgrounds, short lifetimes, and limited concentrations. Here, we use a wide range of complementary X-ray spectroscopies coupled to DFT calculations to study Co(III)4O4 cubanes and their first oxidized derivatives, which provide insight into the high-valent Co(IV) centers responsible for the activity of molecular and heterogeneous OER catalysts. The combination of X-ray absorption and 1s3p resonant inelastic X-ray scattering (Kβ RIXS) allows Co(IV) to be isolated and studied against a spectroscopically active Co(III) background. Co K- and L-edge X-ray absorption data allow for a detailed characterization of the 3d-manifold of effectively localized Co(IV) centers and provide a direct handle on the t2g-based redox-active molecular orbital. Kβ RIXS is also shown to provide a powerful probe of Co(IV), and specific spectral features are sensitive to the degree of oxo-mediated metal-metal coupling across Co4O4. Guided by the data, calculations show that electron-hole delocalization can actually oppose Co(IV) formation. Computational extension of Co4O4 to CoM3O4 structures (M = redox-inactive metal) defines electronic structure contributions to Co(IV) formation. Redox activity is shown to be linearly related to covalency, and M(III) oxo inductive effects on Co(IV) oxo bonding can tune the covalency of high-valent sites over a large range and thereby tune E(0) over hundreds of millivolts. Additionally, redox-inactive metal substitution can also switch the ground state and modify metal-metal and antibonding interactions across the cluster.

In situ characterization of cofacial Co(IV) centers in Co 4 O 4 cubane: Modeling the high-valent active site in oxygen-evolving catalysts

“…The increase in ν max for 1c compared with 1b is evidence for an increased λ accompanying double oxidation of the cubane as supported by DFT calculations (vide infra). The k ET s are on the order of 10 12 s -1 , consistent with Co(IV) in 1b appearing delocalized by electron paramagnetic resonance (timescale of ∼10 −9 s) (21) and localized by X-ray absorption spectroscopy (timescale of ∼10 −15 s) (30). In line with this result, the spectroscopy of the oxidized cubanes is consistent with Robin-Day class II mixed valency.…”

“…An IVCT transition is maintained upon the oxidation of 1b to 1c. The IVCT of the in situ, electrochemically generated 1b is slightly shifted from that of the chemically oxidized species previously reported (30), which may be due to ion pairing effects from electrolyte ions, the presence of trace water in the in situ experiment, incomplete conversion of species within the spectral window, or electric field effects from the Pt flag working electrode. This difference does not preclude a direct comparison between 1b and 1c generated under the same conditions in situ.…”

“…Isolated samples of 1b exhibit an intervalence charge-transfer (IVCT) band at ∼4,580 cm -1 (30). This near-infrared (NIR) absorption is reproduced in the spectroelectrochemical oxidation of 1a to 1b, as shown in Fig.…”

“…Although the ligand field of Co 4 O 4 cubanes differs slightly from that of Co-OEC arising from the presence of pyridine N and acetate O donors, cobalt-oxo molecular species that stabilize one or more Co(IV) centers are rare. The Co(III) 3 Co(IV) state of the cubane has been shown to be a Robin-Day class II mixed-valence (MV) complex (30), and an increase in formal oxidation state to the doubly oxidized cubane has been shown to be active for OER (31,32). In a Co 4 O 4 (py) 4 (OAc) 4 cubane (1) (33), the doubly oxidized Co(III) 2 Co(IV) 2 core may be electrochemically accessed (30).…”

Cobalt‐Bridged Ionic Liquid Polymer on a Carbon Nanotube for Enhanced Oxygen Evolution Reaction Activity

“…Near edge X‐ray absorption fine structure (NEXAFS) spectra of the different samples are given in Figure 2 d. The L3 edge between 775 eV and 785 eV of the spectra is due to the excitation of Co 2p core electrons to unoccupied 3d orbitals of Co 2+ [2p 6 3d 7 →2p 5 3d 8 for Co 2+ ] 15. The first peak of CoSSPIL/CNT‐0.5 (777.3 eV, black vertical line in Figure 2 d) is attributed to t 2g ‐holes3a which exist only in an octahedral environment of Co 2+ . With the increase of PIL content, the t 2g ‐hole features of CoSSPIL/CNT‐1 decrease.…”